Not applicable.
The present invention relates to an arrangement in a multicylinder dryer of a paper machine, which arrangement includes at least one drying cylinder and at least one roll located at a different height to the cylinder and at a distance to it in the machine direction, the paper web traveling from the drying cylinder to this roll and, in which arrangement dry air is brought to the dryer through a beam located in front of the paper web in the machine direction, in which dryer the air is directed by means of nozzles arranged in the beam into a throat between the drying cylinder and the paper web, mainly to intensify the drying event, while a doctor is arranged in, or in connection with the said beam, to keep the surface of the drying cylinder clean.
In the drying section of a paper machine, blow boxes, i.e. ventilators, are used as components that both intensify drying and improve runnability. Doctor devices are used in the drying section to keep the cylinders clean. Generally these devices are located in pockets between the drying cylinders and rolls, so that the doctor is attached to its own frame above the pocket while the ventilator forms its own box beneath this in the lower part of the pocket. In an inverted drying group, these positions are reversed. In this connection, the term roll refers, for example, to a vacuum roll, or to a roll in which a suction effect is induced on the surface of the roll by some other means.
A vacuum, which creates a flow of air moving from the edges of the web to its center, arises in the throat opening between the drying cylinder and the wire. This creates flutter in the edges of the paper web in the area between the drying cylinder and the roll, so that the web lifts off the drying cylinder and may even begin to follow the surface of the drying cylinder. To eliminate this phenomenon, it is advantageous to fill the vacuum by directing a jet of air produced by the ventilator into the open throat. In addition, ventilation can be used to reduce the moisture difference between the edges and the center of the web, thus improving the web""s cross-direction moisture profile. For this to operate as intended, the ventilator""s air jet must be oriented correctly and the permeability of the drying wire and the amount of air blown must be correctly dimensioned.
At present, the doctor equipment is located in the lower pocket of the drying cylinder, above the blow box, attached to its own separate frame. The doctor blade, which is loaded against the drying cylinder, is attached to its holder frame. The doctor equipment is oscillating backwards and forwards in the crossdirection of the machine against the surface of the drying cylinder, from which it cleans impurities adhering to the cylinder.
In the case of wider dryers and faster paper machines, the separate doctor frame and ventilator fill the pocket below the drying cylinder so tightly that, at present, detrimental compromises in certain important functions are inevitable. The replacement air brought by the ventilator cannot be directed effectively enough into the throat opening between the drying cylinder and wire, so that the detrimental vacuum formed there creates transverse air currents that cause flutter at the edges of the web. If the doctor frame is located close to the drying cylinder, temperature differences arise over its cross section, resulting in warping of the doctor frame and poorer blade contact against the drying cylinder. In addition, a reliable and safe attachment location is required for the sensitive control and measurement sensors. In its present unsatisfactory location on a tube beam fitted on the outer circumference of the roll, the web-monitoring sensor may collect paper shreds that cause it to transmit incorrect information on the state of the running of the web. Paper shreds collecting on top of the sensor will indicate that the web is in place, even though the web is broken at, or before the point in question. The unsatisfactory shape of the ventilators causes them to collect broke during a web break, making their blockage a maintenance problem by requiring repeated cleaning. Lumps of wet paper collecting on a ventilator during a break can, in the worst case, damage the drying wire.
Patent publication FI-62571 (U.S. Pat. No. 4,416,070) discloses a combined doctor-ventilator. This method of attaching the doctor blade is characterized by its frame being located very close to the hot drying cylinder. This causes the doctor frame to deflect due to the temperature differences, thus leading to poorer contact of the blade against the drying cylinder. The dynamic pressure effect of the blown air is directed to a narrow area in the throat opening between the drying cylinder and the paper web. In addition, the arrangement has no suitable places in which the sensitive web-monitoring and measurement sensors can be attached safely. The shape of the doctor frame is also such that it readily collects paper shreds and broke.
In the doctor disclosed in the publication, the doctor blade is attached directly to a doctor beam, which is turned between a maintenance position and an operating position. The loading profile of the doctor beam must remain uneven.
U.S. Pat. No. 5,881,472 discloses a concept, in which a current of air is blown into the opening throat by means of a separate ventilator. A separate doctor frame is used to direct the airflow into the opening throat. However, this solution has many corners and, the components are separate from each other, they easily collect paper shreds and broke. This solution too has no advantageous locations for web-monitoring and measurement sensors. The directing of the ventilation is also poor, as the inconvenient location of the components prevents the formation of suitable baffles, which would allow the dynamic pressure effect to be effectively directed at the opening throat.
The invention is intended to create a new type of arrangement for directing ventilation air into the throat opening between the drying cylinder and the paper web and to improve the unimpeded removal of paper shreds into the basement in the case of disturbances in operation. The arrangement according to the invention can be used to achieve a significantly improved drying event. Dry hood replacement air is blown closer and more precisely directed into the opening throat, allowing edge flutter caused by transverse air currents to be prevented by adjusting the amount of air. Besides runnability, drying is also significantly improved, as the web surface is ventilated over a greater distance than at present in the area between the drying cylinder and the roll. In the arrangement, a doctor apparatus disclosed in the applicant""s FI publication 105577, fitted on top of the doctor beam of the drying section, is preferably used as the doctor, so that the holder frame of the doctor forms a smooth continuation to the combined structure, directed to the throat. Loading devices, which are separate from the beam""s operating devices, are provided for the doctor blade. The beam itself can also be moved farther from the hot drying cylinder while, on the other hand, it becomes possible to provide adjustment of the doctor-blade load, which has not been previously used in the drying section.
The doctor is arranged jointly with the roll to form a smooth, high channel for the passage of the web during disturbances, at least on the side next to the roll. The height of the channel is many times greater than the width of its projection.
The doctor blade contact in the edge area is also improved, because the structure supporting the doctor blade is not subject to deflection due to temperature differences. This is of particular benefit in tail threading. Equipment costs drop while the drying effect increases. The improved drying effect of the ventilation brings savings in the drying section by reducing the number of drying cylinders and rolls. The beam""s advantageous smooth shape completely eliminates, from the area of the lower pocket of the drying cylinder, surfaces that might collect broke. Preferably the profile of the beam is high in relation to its width. At the same time, more space than before is reserved for the passage of the web. Safe spaces, where they cannot collect broke or be damaged, are now reserved for the web-monitoring and moisture-profile measuring sensors.
In the following, the invention is examined in greater detail with reference to the accompanying drawings depicting the invention, which is not restricted to the details shown.